1. Field of the Invention
The present invention relates to dense fluid cleaning. More particularly to the use of a dense fluid such as carbon dioxide in a process of centrifugal phase shifting separation as an environmentally sound alternative to organic solvents.
2. Related Art
Liquid-phase phase carbon dioxide cleaning devices and processes can be found in the art. However, none utilize a dense fluid centrifuge or centrifugal processes similar to those detailed herein.
Carbon dioxide exists as a low-density gas at standard temperature and pressure conditions and possesses phase boundaries with a triple point (Solid-Liquid-Gas co-exist in equilibrium like a glass of ice cubes and water) and a critical point (Liquid-Gas have identical molar volumes). Through pressure or temperature modification, carbon dioxide can be compressed into a dense gas state.
Compressing carbon dioxide at a temperature below its critical temperature (C.T.) liquefies the gas at approximately 70 atm. Cooling liquid-state or gas-state carbon dioxide to its freezing point causes a phase transition into solid-state carbon dioxide. Compressing carbon dioxide at or above its critical temperature and critical pressure (C.P.) also increases its density to a liquid-like state (5), however there is a significant difference between compression below and above the critical point.
Compressing carbon dioxide above its critical point does not effect a phase change. In fact, carbon dioxide at a temperature at or above 305 K (88 F.) cannot be liquefied at any pressure, yet the density for the gas may be liquid-like. At the critical point the density is approximately 0.47 g/ml. At or above this point carbon dioxide is termed a supercritical fluid (SCF). Supercritical carbon dioxide can be compressed to a range of liquid-like densities, yet it will retain the diffusivity of a gas. Continued compression of supercritical carbon dioxide causes continued increase in density, approaching that of its liquid phase.
Carbon dioxide is but one of the compounds which is adequate for use as the dense fluid in the within invention other compounds exhibiting suitable dense fluid properties include propane, butane, sulfur hexafluoride liquid nitrogen and liquid ammonia. Those skilled in the art will recognize that without exceeding the intended scope of this invention other compounds exhibiting similar dense fluid properties may be substituted for use in the herein described apparatus and processes.
The use of conventional dense fluid cleaning technology may result in a pooling and supercooling of liquid carbon dioxide trapped within pores and cavities of a substratexe2x80x94leading to the formation of dry ice and recontamination of substrate. The process and apparatus to perform dense fluid centrifugal separations described herein results in precision cleaning using one unit volume of dense fluid per cleaning operation and ability to remove small insoluble particles from deep voids or cavities without dry ice formation or recontamination of the substrate Accordingly, the within dense fluid cleaning and separation apparatus and process overcomes limitations of conventional dense fluid technology and may provide an environmentally-safer cleaning and finishing alternative to organic solvents.
Definitions
The physicochemical cleaning processes and devices described herein are unique to this development field, each exhibiting distinctly different mixing and separation phenomenon. As such, unique terms have been invented herein to describe these processes and devices and are given below:
Dense Phase Carbon Dioxide is used herein to describe all phases of carbon dioxide: liquid state, supercritical state, dense gas state, and solid-state. These states have densities that are within the range of liquid-like or near-liquid substances.
Dense Fluid Centrifugal (Centripetal) Process A process whereby the substrates are rendered immobile under a variable centripetal force which is greater than the gravitational force, and are moved bi-directionally about a central axis in a rotatable drum in predominantly the vertical plane at a rotational velocity which is sufficient to prevent the substrates from mixing within the centrifuge compartment (the rotational velocity necessary is dependent upon centrifuge diameter and weight of substrates), but allows the dense fluid to flow freely at high fluid shearing velocity around and through the substrates. The Dense Fluid Centrifugal process described herein creates two dense fluid zonesxe2x80x94(1) a turbulent cleaning zone located from the center of the centrifuge to outside of the centrifuge drum and a (2) non-turbulent separation zone located about the perimeter of the centrifuge wherein gravitational forces move separated contaminants circumferentially to the lower half for subsequent removal from the centrifuge through a drain port.
Dense Fluid Barreling Process: A process whereby the substrates are mixing bi-directionally under predominantly gravitational force, and are sliding over one another, predominantly in a segmented upper layer, as the barrel rotates slowly about a central axis in a rotatable drum in a plane between vertical and horizontal planes. The rotational velocity is maintained purposefully slow to prevent the substrates from damaging one another during mixing within the barrel compartment (the rotational velocity is dependent upon barrel diameter, weight and fragility of the substrates), but allows the dense fluid to flow freely at lower fluid shearing velocity around and through the substrates. The Dense Fluid Barreling process described herein creates only one dense fluid zonexe2x80x94(1) a semi-turbulent cleaning zone located below the center of the barrel compartment to the lower half of the barrel.
Dense Fluid Tumbling Process: A process whereby the substrates are mixing bi-directionally under predominantly gravitational force, and are sliding over one another, predominantly in a segmented upper layer, as the barrel rotates slowly about a central axis in a rotatable drum in the horizontal plane. The rotational velocity is maintained purposefully slow to prevent the substrates from damaging one another during mixing within the barrel compartment (the rotational velocity is dependent upon barrel diameter, weight and fragility of the substrates), but allows the dense fluid to flow freely at lower fluid shearing velocity around and through the substrates. The Dense Fluid Tumbling process described herein creates only one dense fluid zonexe2x80x94(1) a semi-turbulent cleaning zone located below the center of the barrel compartment to the lower half of the barrel.
Centrifugal Froth Flotation and Separation Process: A process whereby contaminant removal from within voids, cavities and interstitial layers is greatly enhanced by the combined scouring, cavitation and shearing phenomenon produced by the presence of gas-liquid interphases, gas-solid interphases and a variable and bi-directional centripetal force.
Centrifugal Phase Shifting Separation Process: A process whereby contaminant removal from within voids, cavities and interstitial layers is greatly enhanced by the combined scouring, cavitation and shearing phenomenon produced by the isobaric and isothermal exchange of saturated dense fluid vapor and saturated dense fluid liquid, under vapor-liquid equilibrium conditions, and simultaneously in the presence of a variable and bi-directional centripetal force.
The present invention selectively controls the phase change of carbon dioxide between a xe2x80x9csolventxe2x80x9d phase (saturated liquid phase) and xe2x80x9cnon-solventxe2x80x9d phase (saturated vapor phase) at relatively constant pressure and temperaturexe2x80x94isothermal and isobaric change. Precipitated contaminants are separated from a substrate contained in a turbulent xe2x80x9ccentrifugal zonexe2x80x9d of the dense fluid centrifuge by centripetal force and xe2x80x9ctransportedxe2x80x9d by centripetal force to a non-turbulent and circumferential xe2x80x9cseparation zonexe2x80x9d located at the walls of the dense fluid centrifuge. The present invention uses a process of controlled phase shifting no need to transfer contaminated fluid out of the cleaning vessel to prevent redeposition onto substrates.
Contaminants which are selectively soluble under one phase, pressure or temperature conditions and become insoluble when these conditions change may be separated out. The phase shift and separation processes are performed while variable centripetal forces, vortexing forces, and fluid shearing forces are simultaneously acting upon the substrate-contaminant-dense fluid system. The substrate is continuously experiencing a range of cleaning, scouring, washing and separation forces during the process.
A dense fluid centrifuge is also illustrated which can accommodate heavier and eccentric substrate loads, higher rotational velocities, different centrifuge drum designs, spray-under-immersion operations, and liquid-liquid extraction capability. It may also be oriented between the horizontal and vertical planes for improved process capability, system versatility and cleaning performance. Novel processes including centrifugal froth flotation and liquid-liquid extraction are also detailed used as in-situ adjunctsxe2x80x94enhancing additive mixing with carbon dioxide, extracting metallic fines/chips from deep dead-end holes of machined products, scouring particles from filaments of fabrics and performing cleaning of devices not possible using liquid carbon dioxide alone such as stripping plastic coatings from surfaces.
Another feature of the invention is a novel process for simultaneously contacting contaminants on substrates with a primary washing agent containing one or more additives and secondary dense fluid carbon dioxidexe2x80x94liquid-liquid and liquid-supercritical fluid extraction processes. A sequential cleaning operation is carried out in which powerful low viscosity solvents dissolve contaminants and contaminants and solvents are effectively transported away from the objects being cleaned. During such processes, a combination of cleaning actions may be producedxe2x80x94co-solvency, multi-phasic washing and froth flotation (during dense fluid phase depressurization cycles)xe2x80x94during simultaneous and continuous bi-directional centrifugal scouring of substrates in planes from vertical to horizontal.
The present invention also teaches a multi-ported spray manifold which is used for three operations, including (1) prewash spray-under-immersion and froth flotation operations, continuous filtered spray-under-immersion during centrifugal dense fluid extraction and froth flotation operations and (3) post-process substrate heat-up cycle.
Separation of insoluble contaminants (chips, particles, precipitated or reacted soils) is achieved by exerting a continuous and bi-directional centripetal force on substrate-contaminant-dense fluid system during these various operations in-situ. Tumble-cleaning operations may be performed in the present invention by rotating the dense fluid centrifuge into any position from vertical to the horizontal plane for the purpose of mixing, blending, polishing and deaggregating nested substrates. Tumble cleaning used in the present invention is primarily designed for textiles or xe2x80x9cfixturedxe2x80x9d metal substrates during prewash operations. Barrel cleaning is a slower and angled rotation which is adjustable to any angle from full vertical to full horizontal, used for polishing and cleaning.
Also noteworthy is the minimization of the explosive decompression of polymers through the centrifugal wringing action of the centrifuge in combination with a phase shift operation which exchanges liquid carbon dioxide with vapor having much lower molar volume (density) which in-turn reduces the risk of the explosion of the polymers during the expansion of liquid carbon dioxide allowed to remain in contact with the polymer.
Other features and advantages of the present invention will be set forth, in part, in the descriptions which follow and the accompanying drawings, wherein the embodiments of the present invention are described and shown, and in part will become apparent to those skilled in the art upon examination of the following detailed description taken in conjunction with the accompanying drawings, or may be learned by practice of the present invention. The advantages of the present invention may be realized and attained by means of the insturmentalities and combinations particularly pointed out in the appended claims.